1. Field of the Invention
This invention relates to a method for starting up a distilling column which is fated to handle a solution containing an easily polymerizing compound, and more particularly to a method for starting up a distilling column which supplies a polymerization inhibitor to the bottom liquid of the column in a state prior to the stationary state, thereby preventing the column itself and the devices attached thereto from developing polymerization and clogging, and securing safety of itself as well.
2. Description of Related Art
Such easily polymerizing compounds as acrylic acid and methacrylic acid are raw materials for the manufacture of commercial products and are chemical substances which are produced in large quantities at plants of a large scale. In the case of (meth)acrylic acid, for example, this easily polymerizing compound is produced by the reaction of catalytic gas phase oxidation of propylene, isobutylene, t-butanol, methyl-t-butyl ether, or acrolein. In the reaction gas obtained by the reaction of catalytic gas phase oxidation mentioned above, the (meth)acrylic acid as the target product remains mixed with other by-products. For example, the reaction mentioned above mainly generates non-condensable gases, namely unreacted propylene and acrolein, low-boiling organic compounds having lower boiling points than acrylic acid, namely steam and unreacted acrolein, impurities such as formaldehyde and acetic acid which are formed by a secondary reaction, high-boiling compounds having higher boiling points than acrylic acid, namely maleic anhydride, furfural, benzaldehyde, benzoic acid, and acrylic acid dimer, and the like. For the purpose of obtaining the target product by refining this reaction gas, therefore, it is customary to extract the product by counterflow washing the reaction gas with water or a heavy solvent and then supply the extracted product to a distilling column and refine it therein.
Since the distilling column contains therein the target product, solvent, generated gas, etc. in a mixed state, however, it has a high possibility of entailing combustion and explosion and causing the attached devices to sustain breakage. Particularly, the distilling column which handles an easily polymerizing compound further aggravates this possibility because it contains a multiplicity of components and suffers the composition in the column to vary every moment from the time the column starts its operation to the time the column reaches the stationary state. Further, when the target product is an easily polymerizing compound, it is liable to generate a polymer because of the physical properties of its own. When a gas containing molecular oxygen is supplied to the column with a view to preventing this generation of the polymer, the possibility of the supplied gas inducing explosion is heightened.
The starting method which is usually resorted to with a view to preventing such harmful effects will be explained below with reference to FIG. 5. First, for the purpose of preventing a distilling column (1) from explosion, an inert gas is supplied into the interior of the distilling column (1) through the top thereof or the bottom thereof till the entrapped air is displaced with the inert gas and the water entrapped in the column is removed in consequence of the displacement with the inert gas. Then, the initial liquid is introduced into the distilling column via a feed orifice (2) and a reboiler (3) connected to the bottom of the distilling column (1) is heated to start temperature elevation. As the temperature of the bottom liquid is elevated in consequence of this temperature elevation, the low boiling substance, easily polymerizing compound, and high boiling substance are sequentially gasified through the surface of the liquid in the order mentioned. Then, by the temperature elevation, the distillate to a condenser (4) attached to the distilling column (1) is started and the total reflux operation is carried out via a pump (6) by increasing the load to the set amount of reflux by distillate. After the total reflux operation has been stabilized and has been confirmed to bring no adverse effect on the temperature and the pressure inside the column, the supply of a gas containing molecular oxygen to the reboiler (3) is started and, at the same time, the supply of the raw material liquid to the interior of the distilling column (1) is started. Then, the distillate from the condenser (4) attached to the top part of the distilling column is started when the amount of the distillate has increased and the extraction of the bottom liquid is started after the temperature of the bottoms has risen past the present level. Part of the expelled liquid may be circulated via a pump (5) to the distilling column. After the amount supplied has reached the total set level, the stationary operation is started. During the course of this process, the operation is shifted to the stationary status simultaneously with the work of checking the generation and effect of the thermal stress by expansion due to introduction and circulation of the liquid for starting operation, checking the leakage due to application of pressure, checking the heating devices for operation, checking measuring devices for operation, and adjusting the raw materials at the time of charging the reaction vessel of the raw materials.
When the conventional method is adopted for starting up the distilling column and the target for purification happens to be such an easily polymerizing compound as (meth)acrylic acid, the reboiler, the condenser, and the wall and the bottom part of the distilling column are liable to generate a polymer during the course of temperature elevation. When this polymer adheres to the interior of a strainer in the extracting pump stemmed from the bottom part of the column, the adhering polymer has the possibility of giving rise to cavitation and bringing the pump to a stop and preventing the distilling column from continuing a safe operation.
The adhesion of the polymer to the strainer and the clogging of the strainer with the adhering polymer result in requiring the strainer to be given a cleaning work. Owing to the chemical stimulations caused by the raw material for the reaction, the product of the reaction, and the by-products and the physical hindrances caused by the adhesion of a polymer, this cleaning work imparts spiritual displeasure to the workers and compels the workers to suffer adverse effects on health. Further, the organic solvent to be used for distillation entails the problem of jeopardizing the safety of operation on account of the inflammability of the solvent. It is generally difficult to discard manually the spent organic solvent, with the size of the device to be used for cleaning and the size of the distilling column itself to be cleaned as contributory factors.
Moreover, the distilling column of this nature demands a meticulous care and calls for due time and labor when it is started up as well as it is stopped. When the polymer adheres to the distilling column during the operation of starting up the distilling column, since this polymer persists even during the course of the stationary operation of the column, it forms the core of polymerization, gradually accumulates, tends to induce further polymerization and clogging, and forms the cause to stop the continuous operation. When the operation of the distilling column and the operation of the device attached to the column are stopped because of the generation of the polymer, however, the stop of the operation of the large plant and the restart of this operation call for much time and labor. Even the partial stop of the device entails the necessity for adjusting the series of reaction conditions, with the result that the purpose of the quantity production aimed at will not be fulfilled because of the decrease in the productivity.
Further, the removal of the polymer requires to use a large amount of a solvent for detergence. When the spent solvent is discarded or disposed of as by combustion, for example, this disposal entails generation of carbon dioxide, pollution of a nearby river or coast, and failure to attain environmental protection.
When the molecular oxygen-containing gas is supplied prior to or simultaneously with the supply of a heat source for the purpose of precluding the polymerization of an easily polymerizing compound, the gas composition in the column has the possibility of inducing explosion or combustion because it falls in the explosion limits, with the result that instability will be further promoted because of the variation in the composition of the raw materials to be supplied within the interior of the distilling column and the safe start of the column will be rendered more difficult.
Further, even when the polymerization inhibitor is used in the liquid raw material to be supplied for the purpose of precluding the polymerization or the clogging caused by an easily polymerizing compound, since the vapor of the easily polymerizing compound to be generated in consequence of temperature elevation contains no polymerization inhibitor of a high boiling point, no fully effective prevention of the polymerization is attained because the polymerization tends to be induced by the condensation of the easily polymerizing compound.
The present inventor, as a result of an elaborate study concerning the state of a distilling column at the time of starting up the operation thereof, has found that the polymerization and the clogging of an easily polymerizing compound can be precluded by effecting the supply of a polymerization inhibitor to the initial liquid before the arrival of the operation at the stationary state and supplying the molecular oxygen-containing gas and the initial liquid to the prescribed site. This invention has been perfected as a result. To be specific, the tasks imposed on this invention as described above are accomplished by the following method.
A method for starting up a distilling column to handle an easily polymerizing compound-containing solution, characterized by supplying at the start of the operation of the distilling column a polymerization inhibitor to the bottom liquid of the distilling column having a temperature of not higher than 80xc2x0 C.
According to this invention, by supplying the polymerization inhibitor to the bottom liquid of the distilling column having a temperature of not higher than 80xc2x0 C. in starting the operation of the distilling column to handle an easily polymerizing compound-containing solution, it is made possible to preclude the polymerization of the easily polymerizing substance. Particularly when there flux liquid is supplied from the top of the distilling column or from the middle stage of the column before the temperature elevation of the distilling column is started, the polymerization of the easily polymerizing substance can be prevented more effectively because the inner wall of the column is filled with the reflux liquid. Further, by supplying an inert gas and/or the molecular oxygen-containing gas to the interior of the column till the molecular oxygen-containing gas concentration in the column reaches a level in the range of 0.1-9 vol. % before the start of the temperature elevation of the distilling column and then, during the course of the subsequent temperature elevation, supplying the molecular oxygen-containing gas in such a rate that the easily polymerizing compound gas composition within the column exceeds the upper explosion limits, the method for starting up the column is enabled to preclude the occurrence of the polymer of the easily polymerizing compounds within the distilling column and prevent the polymer from adhering to the column and causing clogging and, at the same time, prevent the interior of the distilling column from falling in the explosion limits, and enjoy safe operation.